Switchable current generator

ABSTRACT

A current generator comprising switchable circuit means for selectively establishing current loops for providing high stable null, and equal but opposite polarity currents dependent only on resistive means.

PAIiNlinmzamu FIG.2I

FIG.

I ual current source if the desired SWITCHABLE CURRENT GENERATOR BACKGROUND OF THE INVENTION 7 I Prior, art devices for Supplying null Currents essentially comprised a,+l current source and a -I current source in combination with a diode bridge. The oppositepolarity currentlsources are respectively connected toione'of the two apexes of the diode bridge for 'defining first and second diagonals'thereof and oneof the sources is also connected to a'control signal source while the otherapex of the diode bridge provides an output terminal. In accordance with the input control signal polarity' or level, one'of the H or sources supplies a current to the output terminal of thecircuit.

I One of the disadvantagesof this type of arrangement is the requirement of having two perfectly matched current sources, i.'e., two currentsources capableof supplying opposite polarity "currents of identical amplitude. Obviously, it is extremely'difficult to fabricate and control integrated circuit devices capable 'of providing this precise currenta'mplitude." I

Another disadvantage of the prior art devices resides in the fact that it is necessary to. rebalance' each individoutput current from the circuit is'to be varied.

-' SUMMARY OF THE INVENTION 1 It is thereforeanobject of the present invention to provideatcurrent generator which does not require a precisely balanced pair of current sources.-

Another object of the present invention is to provide a current generator capable of furnishing precise amplitude values of current, +I and I, dependent only I upon a resistive value.

Another object of the present invention is to provide a current generatorwhich is readily implementable into integrated circuit-form'since'the overall circuit does not require precisely balanced-individual current sources. I g

The objectsof the present inventionare obtained by base of a transistor T1 whose emitter and collector iterminals are "connected to thea'node-of a diode'Dl and to a +V voltage source, respectively. The cathode of diode D1 is connected to a current sink schematically represented by CS1 which itself is connected to a voltage source -V. The current sink CS1 draws a current Io. from a node .M which isconnected between the anode. terminals of;a pair of diodes D2 and D3. The anode terminal of diode D2 is connected to a reference =potential, ground-potential in the preferred embodiment. The anode terminal of diode D3 is connected to a node M. Diodes D2 and D3 provide a biasing circuit for node M. The node M is connected to the upper terminal of a'resistive element R and to the anode terminal of diode D4. 'The cathode terminal of D4 is'con- .nected-to the output terminal S of the current generator. The output terminal S is connected to thecathode terminal of diodeDS and its anode terminal is con- .nected to node N and to the lower terminalof resistor R. The node N) is connected to the cathode of diode D6 and its anode is connected to node N andto the anode of diode D7. The cathode terminal of diodeD7 elements CS2 and CS1, respectively.

providinga pairof unidirectional or diode devices re- J spectively connected between first and second nodes and a common output'terminal. An impedance device is connectedacross the first and second nodes whereby current is directed away from the first node for rendering its associated first diode device non-conductive. in

response to a first control signal, and current is suppliedto thesecond node in order to render the second diode device non-conductive in responseto a second control signal for generatingcu'rrents of equal but opposite amplitude.

The foregoing and-other objects, features and advantages of the inventionwill be apparent. from the following more particular description-and preferred embodiments of the invention as illustrated in the accompanying drawings.

BRIEF DESCRIPTION or THE DRAWINGS FIGS. 1, 2 and ,3 schematically illustrate the identical current generator circuitwith differing directional cur-.

rent loops in orderto explainthc; operationof the'invention.

DEscRIPTIoNoF THEEPREFERRED' EMBODIMENT one input controlfterm'inal is connected tothe The output terminal S is maintained at a fixed-potential arbitrarily selected to be equal to Vo/Z for purposes of the preferred embodiment. Any suitable voltage source such as a resistor bridge (not shown) is capable of providing the precise value of voltage Vo/2.

Operationally, the current generator of the present invention provides either a positive or negative valued current. of identical amplitude or a null current according to the input signals applied'to terminals A1 and A2. Now referring to FIG. 1 in order to illustrate the operation of the curre nt generatona down signal-is applied at-both input terminals Al .and A2.

With a relatively down signal applied atterminal A1,

I transistor T1 isnon-conducting and, ,thus, diode D1 is also nonconducting. Accordingly, cur'rentylo is drawn by the current sink CS1 from node M. The value of this current is derived from the (10 1) current flowing to node N by means of the conducting diode D2 and also from the current I- also flowing to node N by means of conducting diode D3,;

When monolithically implemented on the same starting substrate,diodes D1 and D8 exhibit identical characteristics and, therefore, the forwardly biased diode voltage drop, VD for any oneof the diodes D1 -D8 becomes:

h VD F ano de Vcathode v With diode D2 in a conductive state, its anodeis at approximately ground potential and, thus, its cathode ter minal is at a potential -VD in accordance with the above equation. Diode D3 is also conductive and, therefore, its cathode terminal isalso at a potential of -"VD while its anode terminal is at approximately ground or zero potential, again in accordance with the above equation and, thus, node M is also at ground potential. Similarly, the" cathode terminal of diode D4 connected to output terminal S is at a potential level Vo/2 or in a reverse bias state and, thus, diode D4 is non-conducting. As a result of these conditions, current I circulates through diode D3 and, thus, necessarily through resistor R. I

With terminal A2 also at a low or down level, the diode D8 is conducting and, thus, current I is supplied by the current source CS2 down towards terminal A2 through diode D8. During this condition, node N is at a low or down level which differs from the low level applied to terminal AZ by a value equal to the voltage drop across diode D8. Since the cathode of diode D7 is at a potential value of V0 and its anode is at a down node N is at a potential level'equal to Vo/2 VD. The

value Vo/2 VD is also applied across resistor R since node M is at ground potential during' this condition.

The amplitude of the l current is given by the equation:

The current loops for the application of up or high signals being applied to both input terminals A1 and A2 is illustrated in FIG. 2. With terminal A1 at an up level, transistor T1 is conducting and,thus, diode D1 is also conducting. The potential node M is equal to the up level which is represented by the base to emitter voltage. drop of transistor T1 minus the diode .drop or -Vbe VD; During this condition, the diodes D2 and D3 are non-conducting. Accordingly, current is drawn by the current sink CS1 and is applied by the combination of the voltage source +V through transistor T1 and diode D1.

With terminal A2 at a high level, diode D8 is nonconductive and, thus, current lo suppliedby current source CS2 splits and flows through conducting diodes a l current. With diode D7in a conductive state, the

potential at node N is Vo Vd.

Therefore, the potential at node N is given by the equation Vo VD VD VO since diodes with identical characteristics are formed as part of the present Accordingly, the amplitude of the H current is given by the equation: Y

III V0/2 VD/R lt'is thus seen that in both instances the amplitudes of the +1 current and the l'current supplied by the current generatorcircuit of the present invention are identical. Since the voltage value V0 is fixed, the value of these amplitudes is controlled solely by the value of resistor R which can be implementedin many forms, either monolithically, or as a discrete element if it is desired to provide a variable switchable current generator.

Finally, the operation of a circuit is given for the condition of a lower down level being applied to terminal Al and an up or high signal being applied to terminal A2, and illustrated by the current loops in FIG. 3.

With terminal A1 at a low level, transistors T1 and diodes D1 are non-conducting and, thus, current lo drawn by the current sink CS1 is directed away from or supplied by node M. In this state and with diodes D2 and D3 in a non-conductive state, node M is at a zero or null point. Since the cathode of diode D2 is at a potential level Vo/2 and its anode is at a lower potential, it is reverse biased or non-conducting.

With terminal A2 at an up or high level, diode D8 is non-conductive and current Io issupplied by current source CS2 and flows through conducting diodes D6 and D7. With diodes D6 and D7 conductive, the potential at node N is at a potential value V0. With the cathode terminal of diode D5 at potential V0 and its anode at potential Vo/2, it is non-conductive. With both diodes D4 and D5 in a non-conductive state, no current is supplied to the output terminal of the current generator. As the current generating circuit is symmetrical, a null current can also be obtained by applying a high or up level to terminal Al and a low or down level to terminal A2. Briefly,the following table illustrates the various conditions of the output current at terminal S for various high and low control signals being applied to input terminals A1 and A2, for example, as binary values l and zero, respectively.

Control Al 0 0 l l Control A2 0 l 0 1 Output Current l 0 0 +1 Significantly, the circuit means controlling the circulation or current path constituted by current 10 drawn'by the current sink CS1, which includes transistor T1 and 'diode D1, and the circuit means for controlling the current loop or circulation of current Io supplied by the current'source CS2 and including diode D8 perform the same function and, thus, it is within the scope of the present invention to interchange these means. Moreover, transistor T1 and diode D1 can be replaced by a diode by connecting the anode of the diode to terminal Al and the cathode of the diode to node M. In a similar manner, diode D8 can be replaced by a transistor and a diode connected in series with the transistor collector being connected to the -V voltage source and its base terminal being connected to the terminal A2, and the anode of the diode being connected to node N.

Resistor R can also be replaced by any suitable resistive element in a monolithic implementation and even could be implemented by using the internal resistance of a transistor element. Similarly, diodes DI-D8 can be replaced by a transistor having its collector-to-base junction shorted.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details maybe made therein wherein:

without departing from the spirit and scope ofthe invention. I

What is claimed is: g 1. A switchable current generator comprising:

A. first and second input switching terminals and a pair of first and second circuit means connected'tov said first and second input switching terminals,

B. first and second iode conducting means conspect to said output terminal through said seconddiode conducting means, 1 t t v D. said input terminals being responsive to third 'input signals for providing a null current, E. said first circuit means comprising a first biasing means connected to said first node for rendering said first diode conducting means non-conductive,

F. a current sink connected between said first biasing means and a first voltage source for drawing cur-- i 30 G.'said first circuit means being responsive to a first rentfrorn said first node, and

input signals sink for current sink. 2. A switchable current generator as in claim 1 drawing current from said I wherein said first biasing means further comprises:-

A, a third diode conducting means connected between said first reference potential meansv connectedbetween said first reference potential and a third node common to the said current sink and to .said first input switching terminal for'passing current from said first reference potentialtowards'said third node, and

B. a fourth diode current conducting means connected between said first and third nodes for passing a current from said second node toward said third node. I Y

3, A switchablecurrent generator as in claim v V A. said first circuit means further comprises a first transistor having its collector connected to a second voltage source'and a baseterminal connected to said first input switching terminal for receiving said first input signals, and B. a fifth diode conducting means connected between said emitter of said first transistor and said third node for passing current from said emitter terminal of said first transistor toward said third node, said first transistor being selectively switched in response to said first inputsignals. 4. A current generator as in claim 2 wherein: A. said 'firstcircuit means further comprises a sixth diode conducting element connected between said.

' third node and said first input switching-terminal for establishing a current flow from said first inputswitchingterminalto said third node in response to said first input signals at said first input switching terminal for selectively switching saidfsixth diode conducting means. 5. A switchable current generator as in claim 1 wherein said second circuit means'further include:

A. second biasing means connectedto said second node for'selectively' switching'said second diode conducting means, B. a current source connected between said second biasing means and a second voltage source, said current source supplying a current to said second node, and -C.'a second input switching terminal connected between said current source and said first voltage source, and a current flow being selectively established between said second node and-said second input terminal in response to saidsecond input signals at said second input terminal. 6. A current generator as in claim 5 wherein said second biasing means includes:

' A. a seventh diode conducting means connected between a second reference potential and a fourth node common to said current source and to said second input switching terminal for passing current from said fourth node towards said second reference potential, and B. an eighth diode conducting means connected between said second and fourth nodes for passing current from said fourth node to said second node. 

1. A switchable current generator comprising: A. first and second input switching terminals and a pair of first and second circuit means connected to said first and second input switching terminals, B. first and second diode conducting means connected to an output terminal, C. impedance means connected across said first and second diode conducting means for establishing first and second nodes, and said input terminals being selectively responsive to first input signals for selectively activating said first circuit means for generating a current of a first polarity and amplitude through said first diode conducting means, and said input terminals being responsive to second input signals for generating a second current of identical amplitude but opposite polarity with respect to said output terminal through said second diode conducting means, D. said input terminals being responsive to third input signals for providing a null current, E. said first circuit means comprising a first biasing means connected to said first node for rendering said first diode conducting means non-conductive, F. a current sink connected between said first biasing means and a first voltage source for drawing current from said first node, and G. said first circuit means being responsive to a first input signals sink for drawing current from said current sink.
 2. A switchable current generator as in claim 1 wherein said first biasing means further comprises: A. a third diode conducting means connected between said first reference potential means connected between said first reference potential and a third node common to the said current sink and to said first input switching terminal for passing current from said first reference potential towards said third node, and B. a fourth diode current conducting means connected between said first and third nodes for passing a current from said second node toward said third node.
 3. A switchable current generator as in claim 2 wherein: A. said first circuit means further comprises a first transistor having its collector connected to a second voltage source and a base terminal connected to said first input switching terminal for receiving said first input signals, and B. a fifth diode conducting means connected between said emitter of said first transistor and said third node for passing current from said emitter terminal of said first transistor toward said third node, said first transistor being selectively switched in response to said first input signals.
 4. A current generator as in claim 2 wherein: A. said first circuit means further comprises a sixth diode conducting element connected between said third node and said first input switching terminal for establishing a current flow from said first input switching terminal to said third node in response to said first input signals at said first input switching terminal for selectively switching said sixth diode conducting means.
 5. A switchable current generator as in claim 1 wherein said second circuit means further include: A. second biasing means connected to said second node for selectively switching said second diode conducting means, B. a current source connected between said second biasing means and a second voltage source, said current source supplying a current to said second node, and C. a second input switching terminal connected between said current source and said first voltage source, and a current flow being selectively established between said second node and said second input terminal in response to said second input signals at said second input terminal.
 6. A current generator as in claim 5 wherein said second biasing means includes: A. a seventh diode conducting means connected between a second reference potential and a fourth node common to said current source and to said second input switching terminal for passing current from said fourth node towards said second reference potential, and B. an eighth diode conducting means connected between said second and fourth nodes for passing current from said fourth node to said second node. 